The sol-gel procedure for the preparation of nickel(!!) hydroxide
micro-spheroids of definite size and shape is described.
The process is a two-stage operation. In the first stage the
concentrated sol of Ni(OHh is produced by a sequence of precipitation,
centrifugation, resuspension and electrodialysis. In the
second stage the sol is transformed into a gel in a column in the
counter-current flow of a dehydrating alcohol. Flow rate control
of the sol and of the dehydrating agent allow preparation of spherical
particles of different diameters in the range of 0.1 to 0.3 mm

D. M. Novak

N. Bonacci

English

The sol-gel procedure for the preparation of nickel(!!) hydroxide

micro-spheroids of definite size and shape is described.

The process is a two-stage operation. In the first stage the

concentrated sol of Ni(OHh is produced by a sequence of precipitation,

centrifugation, resuspension and electrodialysis. In the

second stage the sol is transformed into a gel in a column in the

counter-current flow of a dehydrating alcohol. Flow rate control

of the sol and of the dehydrating agent allow preparation of spherical

particles of different diameters in the range of 0.1 to 0.3 mm

Bonacci, N.

Novak, D. M.

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CROATICA CHEMICA ACTA CCACAA 45 (4) 531-537 (1973) CCA-796 YU ISSN 0011-1643 541.18;546.74 Original Scientific Paper A Sol-Gel Process for the Preparation of Micro-Spheroids of Nickel (II) Hydroxide* N. Bonacci and D. M. Novak Laboratory of Electrochemistry and Laboratory of Physicochemical Separations, »Ruder Bo!ikovic« Institute, 41001 Zagreb, Croatia, Yugoslavia The sol-gel procedure for the preparation of nickel(!!) hy-droxide micro-spheroids of definite size and shape is described. The process is a two-stage operation. In the first stage the concentrated sol of Ni(OHh is produced by a sequence of pre-cipitation, centrifugation, resuspension and electrodialysis. In the second stage the sol is transformed into a gel in a column in the counter-current flow of a dehydrating alcohol. Flow rate control of the sol and of the dehydrating agent allow preparation of sphe-rical particles of different diameters in the range of 0.1 to 0.3 mm. INTRODUCTION The interest in nickel(II) oxide and nickel has been revived in recent years due to their characteristic semiconducting properties and general technological importance, e.g. plating, catalysis, and batteries. Therefore further investigation was encouraged towards the study of their electrochemical properties, as well as to the precise surface and adsorption studies. Nickel(II) hydroxide is used as the starting subsfance to obtain samples of nickel(II) oxide and nickel, through a rather simple heating procedure1•2 of transformation of nickel(II) hydroxide to nickel(II) oxide at temperatures between 250 and 1300 °C3. At higher temperatures (about 1300 °C) nickel is obtained by reduction. Usually, two types of samples were used ~n nickel-nickel(II) oxide studies, either a) single crystals or b) powdered samples. The preparation of single crystals usually involves difficult procedures and complicated apparatus4- 7• The powdered samples of NiO are easily prepared either by mechanical procedures 'such as crushing8 or by different preparation procedures such as evaporation9 or reduction of nickel(II) compounds10• The dimensions of the particles obtained and used were from 2.5 µm to 0.1 µm . N.ickel(II) hydroxide powders are easily prepared by precipitation of nickel(II) compounds, under the conditions described earlier11. However, the disadvantage of the powdered samples is related to the undefined shape and size of particles. Therefore efforts are encouraged to prepare samples of pure nickel(II) hydroxide of definite size and shape. The slow precipitation method for pre-paration of nickel(II) hydrox.ide11 combined with the modified sol-gel proce-dure for uranium(V) hydroxide proposed earlier12 were employed. * Supported by Grant No. GF 31530 (P. L. 480) from the National Science Foundation, Washington, D. C. USA, and in part by a Grant from the Council for Scientific Research of the Socialist Republic of Croatia. 532 N. BONACCI AND D . M. NOVAK EXPERIMENTAL Preparation of nickel(II) hydroxide Nickel(II) hydroxide was prepared by precipitation from aqueous solutions of analytical grade nickel(II) chloride (Hopkin and Williams Ltd., England) and sodium hydroxide (Merck Co., Darmstadt), under the conditions described earlier11. The preparation was based on very slow drop by drop addition (approximately 30 min) of 1 M sodium hydroxide, to a vigorously stirred 0.2 M nickel(II) chloride. The freshly prepared nickel(II) hydroxide suspension was washed four or five times combined with centrifugation with doubly distilled water to remove any excess of chloride ions. Deionisation of suspension The deionisation of nickel(II) hydroxide suspension was carried out by electro-dialysis in a specially designed three compartment cell described earlier11 " 3, with only small modifications. Two plexiglass cells suitable for laboratory preparations were used. The volume of the central compartment was 75 ml and the other 1100 ml. The central compartment was divided from the cation and anion compartments and its platinum electrodes by cation and anion exchange membranes (Zeolit »Perma-plex« C-20 and A-20, Water Softeners Ltd., England). The active exchange area per membrane was 32 and 144 cm2 for the small and the large cell, respectively. The central compartment contained a nickel(II) hydroxide suspension which was con-tinuously stirred while the side compartments contained only slightly alkaline water. The electrodialysis was carried out by employing a constant current source at the maximum current density at the membrane of 0.2 mA cm-2• The electrodialysis usually lasted up to 72 hours after which the specific conductivity of the supernatant liquid was less than 2 x 10-s Q-1 cm-1. Preparation of stable sol Attention was paid to find out suitable conditions at which nickel(II) hydroxide is a stable sol. It was desirable to achieve the stability of the purified sol without any additions of the stabilizing agents recommended in literature1•-rn. Therefore the deionised suspension of nickel(II) hydroxide was heated up to 70 °c and vigorously stirred for at least two hours, always taking care not to allow peptization to occur . The sol was then concentrated up to 1.8 M on a rotating vacuum evaporator thermo-stated at 30 nc. The concentrated sol showed thixotropic characteristics, and ac-cordingly the evaporation was carried out intermittently, the intervals being used for vigorous shaking of the sol to achieve fluidity. Fig. I. is an electron micrograph of a stable sol. Fig. I. Electron micrograph of the stable sol of nickel(II) hydroxide produced at room t empe-rature. The average particle size is 100 A and more. NICKEL (II) HYDROXIDE MICRO-SPHEROIDS 533 Transformation from sol to gel The transformation of nickel(II) hydroxide to nickel(II) hydroxide gel micro--spheroids of definite size and shape distribution occured when a thin jet of con-centrated sol was introduced at the top of a column with the counter current flow of an organic dehydrating solvent. The sol was dispersed to fine particles and gel micro-spheroids were formed during the fall through the column. The apparatus consists of a glass column and a d ispersion system both supplied by peristaltic pumps as shown in Figs. 2 and 3. The dispersion system employed consists of a set of capillaries through which the sol is driven ointo the column. The dimensions of the micro-spheroids were determined by the flow rate of the hydroxide sol through the capillaries and also by controlling the rate of drop detachment. Two dimensions of glass columns were used. The smaller one, height 110 cm and diameter 9.5 cm, for laboratory experiments and the semi-industrial type of a column, height 476 cm and diameter 20 cm. Mixtures of long-chain alcohols a) »Alphanol« 79 (Shell, Germany) - mixed normal primary and a-methyl substituted primary aliphatic alcohols, with some / Fig. 2. Photograph of the apparatus used for preparation of nickel(II) hydroxide micro-spheroids. Fig. 3. Schematic diagram of the apparatus used for preparation of nickel(II) hydroxide micro--spheroids: a) the dispersion system, b) the column and the flow chart: PPR-peristaltic circulation pumps, DPS-sol dosage pump. Insert shows capillaries for drop production. 534 N. BONACCI AND D. M. NOVAK saturated cyclic alcohols and b) »Nonanol« (Shell, Germany) - essentially 3,5,5--trimethylhexanol-1, were used as dehydrating solvents. The particles should float in the organic solvent for at least three hours. The floating of the particles through the column is deter mined by the rate of the counter current flow of the organic solvent which could be easily adjusted. When the drag of the counter current flew is less than the gravitational force acting on the particles, the dehydration is considered to be slow, and the particles form clusters instead of micro-spheroids. Therefore the recirculation should be adjusted to keep the particles floating. Also the choice of dehydrating solvent plays an important role. Better results were achieved when »Nonanol« was used, as its total water absorption is 2 times larger than that of »Alphanol« 79. Gel micro-spheroids thus obtained were washed in petroleum ether and then dried at approx. 10-3 Torr (1 Torr '""" 101.325/760 kPa) for few hours. CHARACTERIZATION OF THE MICRO-SPHEROIDS The nickel(II) hydroxide micro-spheroids obtained by the pr.oced ure descri-bed are speherical particles of a light green colour and are stable in air. The micrograph of the salffiple is given in Fig. 4. 1100 pm .___. Fig. 4. Micrograph of nickel(!!) hydroxide micro-spheroids dried at 10-• Torr in air. The average particle diameter is 120 µm. Chemical analysis of the micro-spheroids An elemental carbon and hydrogen analysis was performed to determine the quantity of the organic dehydrating agent left on the surface of the micro-spheroids. The analysis gave 2.70/o wiw carbon and 2.50/o w/w hydrogen. These impur~ties were removed in further t ransformation procedure of nickel{II) hydroxide to nickel(II) oxide by heating to 700 °c in air, as the further analysis showed 0.2610/o carbon and 0.120/o hydrogen. Preparation of nicke(II) oxide micro-spheroids The preparation w as carried out by employing the heating pr.ocedure pro-posed earlier1•2• The process runs continuously and can easilY. be followed through the change of colour of the sample, as the light green colour, charac-NICKEL (II) HYDROXIDE MICR.0-SPHEROIDS 535 teristic for nickel(II) hydroxide is changed to the grey one of cnikel(II) oxide. The temperature at which the transformation begins was observed to be 230 °C while the transformation is complete at 715 °C. Granulometric analysis of the samples The size distr.ibution of •the particles was determined by granulometric analysis using a s·tandardized sieve set '(Soiltest Inc. , Chicago). Different sizes of the micro-spher.oids obtained are shown in Fig. 5. Infrared analysis and X-ray spectrum Infrared cpectra of nickel(II) hydroxide micro-spehroids and nickel(II) oxide micro-spheroids incorporated iin KBr pellets were run on a Perkin Elmer 137 spectrometer. The characteristc spectrum of nickel(II) hydroxide is observed although the minor CH-impurities are observed as a band at ,.., 3000 cm-1• On the contrary, the spectrum of nickel(II) oxide obtained from nickel{Il) hydroxide micro~spheroids shows that all of impurities are eliminated. The X-ray analysis was carried out on a Philips Difractometer PW 1010/30 with CuKa beam. · The powder method was employed to indicate the degree of crystallinity of the samples prepared. The x -ray analysis of nickel(II) micro-spheroids was compared with the analysis of the crystalline sample of nickel(II) hydroxide19. so~ 1.0 r-30 -20 I I II I I 50 100 150 200 250 300 SIZE OF MICRO SPHEROID 5 I pm · Fig. 5. Diagram showing the particle size distribution after the granulometric classification of nickel(II) hydroxide micro-spheroids. 536 N. BONACCI AND D. M. NOVAK The characteristic reflexes for crystallinity of nickel(II) hydroxide are only poorly evident in the case of our sample, which indicates our sample as amorphous. Determination of specific surface area of microcspheroids The specific surface area of micro-spheroids was determined by argon gas adsorption using the BET method20 • The typical value found was 75 m 2g-1 for particles of 150 to 250 µm in diameter. The complete adsorption . isotherm (Fig. 6) for argon shows essentially an absence of hysteresis and consequently the samples have no micropores. Further adsorptron studies are in progress. so,,----------------~ -0.: ,_; ui 40 ~ 30 0 > 0 UJ CD 0:: 0 <fl 0 20 -cc 10 0 'IOSORPTiON A DESORPTION 0.1 0.2 0.3 0.4 ClS 0.6 0.7 0.8 P/Po Fig. 6. The adsorption isotherm for argon on nickel(II) hydroxide micro-spheroids of 115 to 250 µm in diameter and of a specific surface area of 75 m'/g. CONCLUSIONS Nickel(II) hydroxide micro-spheroids were obtained by employing a rather simple, modified sol-gel procedure. The controlled slow preci!pitation method was preferred over the rather fast preparation procedures proposed earlier11 . Comparatively pure material was obtained in the sol form as no stabilizing agents were added. Electrodialysis has been shown to proceed successfully at a current density at the membrane of 0.2 mA cm-2. Transformation of a stable sol was shown to proceed best in" a column filled with nonanol as dehydrating agent. An advantage of nickel(II) hydroxide micro-spheroids is that the particles are of definite size and have defined specific surface area what ena1bles further use in adsorption and surface irnvesti!gations. The particles are compact and NICKEL (II) HYDROXIDE MICRO-SPHEROIDS 537 micropores are absent which could be an advantage when the pellets of micro-spheroids, or pressed electrodes are prepared. Acknowledgement. We acknowledge the interest shown by Dr. Z. Pucar and Dr. V. Pravdic. We also thank Mr. Z. Stuka and Mr. I. Coch for their technical assistance, and to Mr. F. Matijevac for running the adsorption isotherms. REFERENCES 1. W. D en n st e d t and W. Lose r , Electrochim. Acta 16 (1971) 429-431. 2. S. Le Bi ha n and M. Fig 1 a r z, Electrochim. Acta 18 (1973) 123-124. 3. J. T. Richardson and W. 0. Hi 11 i g an, Phys. Rev. 102 (2) (1956) 1289-1294. 4. H. Dietrich and E. Kn e 11 er, Z. Metallk. 47 (1956) 672-684. 5. Y. Nakamura, J. Hay a sh i, K. Fur u k a n a and M. A s o, J. Phys Soc. J ap. 9 (1954) 204-208. 6. P . R. Row 1 and, Trans. Faraday Soc. 47 (1951) 193-194. 7. R. Sch o 1 de r and E. Gies 1 er, Z . Anorg. Allg. Chem. 6 (1962) 237-246. 8. M. Qua tine t z, R. J. Schafer, and C. Smeal, Trans. AIME 221 (1961) 1105-1110. 9. T. Fur u o ya and Y. Sasaki, J. Phys. Soc. Jap. 17 (1962) 1809-1810. 10. A. C. Zettlemoyer, Yung-Fang Yu, and J. J. Chessick, J. Phys. Chem. 59 (1955) 588-592. . 11. D. M. Nov a k - Ad am i c, B. Cos o vi c, H. Bi 1 ins k i, and M. Br an i ca, J. Inorg. Nucl. Chem. 35 (1973) 2371-2382. 12. Electrochemical Preparation of U02 and the Sol-Gel Precipitation, Annual Report (1971), »Ruder Boskovic« Institute, Zagreb, Croatia, Yugoslavia. i3. I. R. Higgins, J. T. Roberts, C. W. Hacher and J. A. Marin ski, Ind. Eng. Chem 50 (3) (1958) 285-292. 14. W. Feit kn e ch t, R. S in g er and A. B e r g er, Kolloid Z. 101 (1942) 12-20. 15. K. C. Sen and N. R. Dhar, Kolloid Z. 33 (1923) 193-202. 16. M. Pr as ad and V. Swami n at ha n, J . Colloid. Sci. 7 (1952) 26-36. 17. 0 . F. Tower and M. C. Cooke, J. phys. Chem. 26 (1922) 728-735. 18. D. T. Ewing and J. M. Tobin, J. Electrochem. Soc. 103 (1956) 648-651. 19. INDEX, Inorganic Powder Diffraction File, American Society for Testing and Materials, ASTM-14-117, 1969. 20. S. Ross and J. P. 0 1 iv i er, On Physical Adsorption, Interscience, New York 1964. IZVOD Sol-gel-postupak za pripravu mikrosferoida niklova(II) hidroksida N. Bonacci i D. M. Novak Opisan je posturpa:k za priprravu mikrosfe1101da niklova(II) hidro~sida odredene veliciine i oblika. Postupak se sastoji od dViije faze : u prvoj ,se pripravlj,a sol Ni(OH)2 nizom operaci:ja ~talofonje, centrifugiranje, pono,Vino suspendiranje i elektrodijaliza) , a u drugoj se sol pretvara u gel, u kolond kroz koju se u protustruji pusta dehidrira-juci dugolancami al:kohol. Prikladndim ,izborrom ibrzina protaka soLa i dehidrirajuceg sredstva mogu se p11ipraviti sferne cestice raznih promjera, od 0,1-0,3 mm. LABORATORIJ ZA ELEKTROKEMIJU I LABORATORIJ ZA FIZICKO-KEMIJSKE SEPARACIJE. INSTITUT »RUDER BOSKOVIC«, 41000 ZAGREB Primljeno 10. rujna 1973. 

A Sol-Gel Process for the Preparation of Micro-Spheroids of Nickel (II) Hydroxide

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